1. Field of the Invention
The present invention relates to an exhaust structure for discharging exhaust gas after combustion to the outside in an outboard motor on which an internal combustion engine is mounted as a power source.
2. Description of the Related Art
In this type of outboard motor, an engine output torque is transmitted from a drive shaft to a propeller shaft, and a propeller disposed at a rear part of the outboard motor is rotated, to thereby obtain a thrust. The propeller has a cylindrical boss, and is attached to the propeller shaft with this boss portion. Exhaust gas from an engine passes through the boss to be discharged into water.
Generally, a gap between an outside diameter of a front end portion of a propeller boss and an inside diameter of a rear end portion of a gear case cannot be set to a value equal to or less than a certain value for avoiding a contact due to a swing of the propeller. In this case, there is a possibility that exhaust gas passing through an inside of the propeller boss is leaked from the gap. When the leaked exhaust gas is led into the propeller, and if this state continues, a propulsion efficiency of the propeller is decreased.
Accordingly, an outboard motor disclosed in Patent Document 1, for example, employs a structure in which a rear end portion of a gear case or a cover of the gear case is protruded so that an inner periphery and an outer periphery of a front end portion of a propeller boss are overlapped. With such a structure, a leakage of exhaust gas is prevented.
Patent Document 1: Japanese Utility Model Application Publication No. 55-085999
In the outboard motor disclosed in Patent Document 1, although a labyrinth structure is formed between the front end portion of the propeller boss and the rear end portion of the gear case, even in this case, there is required a gap, to no small extent, to prevent a contact between the both due to a swing of a propeller shaft. Accordingly, it was not always possible to achieve a sufficient effect of preventing a leakage of exhaust gas.